Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known to incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices.
Biventricular pacing has been shown to coordinate contractions of the left and right ventricles, reduce the amount of blood flow that leaks through the mitral valve, and decreases the motion of the septal wall that separates the chambers of the heart. Such motion can affect the quantity of blood that the ventricle can pump out in a single beat.
Biventricular pacing has been found to be particularly advantageous in patient's suffering from heart failure disease because of the improved ability of the left ventricle to fully pump blood from the heart. As a result, patients are able to tolerate greater exertion, have a longer life span, and experience a higher quality of life.
With the ability to pace either or both sets of corresponding heart chambers, it is believed that a wide variety of irregular heart conditions may be most efficiently addressed. For example, in a patient suffering from dilated cardiomyopathy, typically the left ventricle is predominately affected in the earlier stages of the disease. The dilated left ventricle has diminished contractility causing its contraction to be slower and weaker than the still healthy right ventricle. Thus, by selecting the stimulation pathway direction from the left ventricle to the right ventricle, the slower left ventricle contraction is initiated prior to the faster right ventricle contraction, yielding superior synchronization of right ventricle and left ventricle contractions.
Pacemaker Mediated Tachycardia (PMT) also called “endless-loop tachycardia”, or “pacemaker reentrant tachycardia”, is a recognized pacemaker related rhythm anomaly. PMT can result in any dual chamber pacemaker capable of sensing and responding to atrial depolarizations when A-V synchrony is dissociated, typically by a premature ventricular contraction (PVC). Ventricular events are conducted in a retrograde direction to the atria that cause atrial depolarizations. The device senses this retrograde atrial depolarization and then, after the appropriate AV delay, delivers a stimulus to the ventricle. Thus, the device provides the antegrade conduction pathway for the reentrant circuit and the intrinsic conduction system of the heart provides the retrograde pathway. A repetitive cycle of ventricular, retrograde P-wave synchronized pacing can ensue.
Once PMT is detected and confirmed it can be terminated by extending the post ventricular atrial refractory (PVARP) long enough such that the retrograde P wave is not tracked and the circuit is broken. Another method for terminating PMT is by restarting the AV cycle, i.e., delivering an atrial pacing output at a fixed time after the retrograde P wave.
Traditional single ventricular output PMT detection methods use the high P-V rate and the stability of the V-P retrograde timing as PMT classifiers. While these methods can successfully detect PMT, improved PMT detection methods are needed for use in pacemakers and defibrillators providing bi ventricular pacing therapy.
In single ventricular pacing modes, the V-P stability classifier relies on the existence of a single PMT retrograde pathway. It is possible however, that in bi ventricular pacing modes, two PMT retrograde pathways can co exist such as in the case of intermittent left/right ventricular captures. Under this condition, the V-P time alternates between the two PMT pathways, one for the ventricular output whose captured signal corresponds to a short retrograde path and another for the ventricular output whose captured signal corresponds to an equal or longer retrograde path. These two PMT pathways may manifest when any one ventricular output intermittently fails to capture allowing the other retrograde path to perpetuate the PMT through a different conduction path.
The traditional PMT classifier that relies solely on a single V-P stability criterion can misclassify a valid PMT and allow a dual-circuit PMT to continue undetected. The result is inappropriate pacing therapy having a pacing-sensing feedback characterized by artificially high ventricular pacing rates. The present invention addresses these and other issues.